Hemidiscus cuneiformis, type of the family Hemidiscaceae, is a diatom with a distinctive semi-circular shape in valve view. A strain of H. cuneiformis was established from the coastal region of the tropical southwestern Atlantic Ocean off Brazil. Cells of this strain showed semi-circular to asymmetrically elliptical valves, indicating morphological plasticity. In the small subunit (SSU) rDNA phylogeny, the type species of Hemidiscus branched between two clades of Actinocyclus, one of which was a sister group, the other in a basal position. Beyond cell shape, there are no significant morphological differences between Hemidiscus and Actinocyclus. We propose the transfer of H. cuneiformis and H. kanayanus into Actinocyclus. The family Hemidiscaceae remains for the genus Actinocyclus.